Chapter 16Evolution of Populations

Variation and Gene Pools• Genetic variations (differences) are studied

in populations (group of individuals of the same species that interbreed).

• A gene pool is all of the genes that are present in a population.

• The relative frequency of an allele is the # of times that the allele occurs in a gene pool compared with the # of times other alleles occur (usually a %).

• In genetic terms, evolution is any change in the relative frequency of alleles in a population.

Sources of Genetic Variation

• There are two main sources of genetic variation:oMutations (any

change in a sequence of DNA)

o The gene shuffling that results from sexual reproduction.

Single-Gene and Polygenic Traits• The number of phenotypes produced for a

given trait depends on how many genes control the trait.

Single-gene trait:Trait controlled by a single gene with two alleles (ex: widows peak)

Polygenic trait:Trait controlled by two or more genes, each gene has two or more alleles (ex: height)

Single-Gene Traits• Natural Selection on single-gene traits can

lead to changes in allele frequencies and thus to evolution.

Polygenic Traits

• Natural selection can affect the distributions of phenotypes in any of three ways:o stabilizing selectiono directional selectiono disruptive selection

Stabilizing Selection• When individuals near the center of the curve

have higher fitness than individuals at either end of the curve

Directional Selection

• When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end

Disruptive Selection

• When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle

Genetic Drift• A random change in allele frequency.• In small populations, individuals that carry a

particular allele may leave more descendants than other individuals do, just by chance.

• When allele frequencies change due to migration of a small group of a population it is known as the founder effect.

16-3 The Process of Speciation

Vocab• Gene Pool• Relative frequency• Single-gene trait• Polygenic trait

o stabilizing selectiono directional selectiono disruptive selection

• Genetic drift• Founder effect